Known existing speech sound enhancement processes include a technology for using characteristics of audio signals and enhancing formants, which are multiple spectral peaks of received speech sound and influence vowel identification, in accordance with the level of ambient noise (see Patent Document 1, for example), and a technology enhancing received speech sound according to the SNR of the received speech sound and the ambient noise (e.g. Patent Document 2).
[Patent Document 1] Japanese Patent Publication No. 4018571
[Patent Document 2] Japanese Laid-open Patent Application Publication No. 2000-349893
However, in the case where the received speech sound is enhanced according to the above conventional technology (Patent Document 1 or 2), the balance between the low frequency range and the high frequency range is lost, and the enhanced sound becomes muffled (a low frequency component is excessively dominant) or becomes shrill (a high frequency component is excessively dominant). Thus, the conventional technologies leave the problem that their enhancement is subject to a reduction in speech sound quality and is not sufficient to make the received speech sound clear.